Conventionally, in the serger, the double chain stitch sewing machine, or the cover stitch sewing machine, etc., the gas carrying threading device which is connected by a hollow looper thread guide which leads to a looper thread guide outlet of a loop-taker point of the looper from a thread introduction portion which inserts the looper thread and which feeds a looper thread by utilizing a flow of a pressurized gas which is supplied to the hollow looper thread guide is known. Hereby, a complicated thread guard is unnecessary and a threading that a handleability is easy can be performed. Therefore, there are no mistake of the threading, no protrusion of the looper thread in mid-process, and no entanglement of the inserted looper thread with other thread, and thereby the threading can be performed at once by the extremely easy operation (Patent document No. 1-No. 3).
In a structure of the foregoing gas carrying threading, a pathway for the threading becomes considerably simple, and an operation of the threading becomes easy, and an entanglement of the thread or an incidence of a thread breakage can be dissolved.
However, in the structure of the foregoing gas carrying threading, while pressing a stopper shaft (positioning pin) for a connecting positioning plate by one hand, a pulley is rotated manually by the other hand, and thereby a stitch forming device must be locked and concurrently a threading connecting device must be connected. Therefore, it is difficult to understand how to use this threading device for the operator who is not familiar with the sewing machine, and an insertion operation of the looper thread which is performed by using both hands concurrently is considerably complex, thereby the training of that purpose is necessary.
Then, a looper threading device which inserts the looper thread to the looper by several operations by one hand without using both hands concurrently and attempts an easy threading operation is proposed (Patent document No. 4-No. 5).